[artemiscloud#28] refactor to reduce code repetition

Currently in ArtemisJolokia code there are many common codes among
fetching methods for different components. For example
readBrokerAttributes, readAddressAttributes and
readClusterConnectionAttributes are all sending 'POST' requests
with similar pattern but only different parameters.

It can be abstracted to a common method shared by those methods
and also will be reused when we add new components like Bridges.

Author: howardgao